1. Field of the Invention
The invention relates electrochromic devices characterized by an optical transparency that varies responsive to an external potential applied between two electrodes. More specifically, it relates to electrochromic devices comprising solid inorganic materials.
2. Background Information
Ionic electrochromic devices comprise two active layers separated by an electrolyte through which a selected mobile ion can be controllably moved from one of the active layers to the other. Both of the active layers can contain varying concentrations of the mobile ion. In at least one of the active layers, commonly referred to as the ‘electrochromic layer’ (‘EC layer’), changes in the ion concentration are accompanied by changes in the color and transparency of the layer. In the other active layer, commonly referred to as the ‘ion storage layer’ (IS layer), changing the ion concentration may or may not be accompanied by notable changes in transparency or coloration. If the IS layer does not show a visible change, it is referred to as a “passive ion storage layer”. If the applied voltage causes an optical change in the IS layer similar to the one occurring in the EC layer, it is called a ‘complementary ion storage layer”. The various layers in an ionic electrochromic device may all be solid materials.
Although many electrochromic devices (ECDs) are of the ionic sort, it is known to make a reduction-oxidation ECD (‘redox ECD’) that operates via molecular oxidation-reduction reactions rather than by moving intercalating ions among various layers within the device. The prior art of redox ECDs teaches devices comprising one or more active layers having either a liquid or gel solution of some sort or another as one of the layers. Generally speaking, the use of liquid and gel solutions limits the stability and operating temperature range of these ECDs. The prior art in redox ECDs does not provide a solid-state ECD consisting of a plurality of thin solid films and having a single solid active layer.